MPAA Rating: PG-13 for language including some sexual material and a drug reference.

The first half-hour of “The Invention of Lying,” co-directed and co-written by Ricky Gervais and Matthew Robinson, is so sharply fresh, clever and laugh-out-loud hilarious that you can’t help but wonder how they’ll sustain it for another hour.

To be honest, they can’t.

The early reels are a marvel of sustained comic invention and verbal wit, with a dash of speculative humor that borders on science fiction. Bus ads and building signage offer priceless sight gags (a retirement home bills itself as “a sad place for old hopeless people”), while the movies Mark writes are essentially unvarnished history lessons, read directly to the camera with nary an ounce of Hollywood artifice.